This invention relates to a device for automatically lapping valve seats and more particularly to such a device suitable for repairing main steam valves and the like used in atmospheres highly contaminated with radioactive substances in nuclear power plants or the like.
Nuclear power plants are subject to legal periodic inspections at predetermined time intervals. Under these circumstances it is frequently required to lap valve seats upon repairing main steam isolation valves and other valves. The lapping operation has be previously necessary to be manually performed. Therefore after a radiation level within the particular nuclear reactor has been reduced to its permissible value or less, repair personnels are usually entered into that nuclear reactor to perform the operation of lapping valve seats within a time interval as short as possible. It has been previously said that a radiation dose of each operation is inversely proportional to the square of a distance from a contamination source involved and also such a radiation dose is proportional to his or her working hours. Therefore the same operator can not work for a long time and the exposure dose of the operator restricts his or her working hours. This has unavoidably led to the shift of repair personnels after short working hours in view of the control of their health and accordingly to the necessity of securing many experts.
On the other hand, devices for lapping valve seats have been previously proposed. Most of the proposed lapping device have been of the manually operated type although some of the devices have been of the power operated type. In either type of conventional lapping devices a valve seat to be lapped has been coated with a lapping agent and centered on the lapping plate. Then the lapping plate has been rotated with a predetermined rotational force and with respect to the valve seat thereby to lap the latter through the lapping agent. However those devices have been of such a structure that a pressure applied to the interface of the valve seat and lapping plate is maintained at a predetermined fixed magnitude and can be adjusted in accordance with the area of the particular valve seat and that for each cycle of the lapping operation the lapping agent is applied to the valve seat being lapped or the next valve seat after the removal of the lapping plate. It is desirable to prove a device for automatically lapping valve seats operated at a remote position. This is particularly desirable for lapping valve seats used in the nuclear reactor because jobs performed within the reactor are not desirable in view of the health of the operators.